Musnad Ahmad 1153:221

Yahya narrated to us, from Hisham, who said: My father informed me, from Aisha, who said: "The day of Ashura was a day that Quraysh used to fast during the Pre-Islamic period (Jahiliyyah), and the Messenger of Allah (peace and blessings of Allah be upon him) used to fast it. When he arrived in Medina, he fasted it and commanded that it be fasted. When the fasting of Ramadan was revealed, Ramadan became the obligatory fast, and Ashura was abandoned. So, whoever wished fasted it, and whoever wished did not fast it."